This application claims the priority of Swiss Application No. 1999 0161/99 filed Jan. 29, 1999, which is incorporated herein by reference.
Packing machines using plastic film wrappers frequently include sealing shoes with cooperating counter shoes for providing sealed seams on superposed plastic films (sheets) forming a packing hose. The sealing shoe or both the sealing shoe and the counter shoe are heated to a temperature above the melting temperature of the thermoplastic packing sheet, and the shoes are pressed to one another to seal the superposed plastic films positioned between the two shoes.
The output capacity of the above-outlined systems is necessarily limited. The heat is conducted through the film into the sealing zone. If the sealing temperature is too low, insufficient heat is transferred to the sealing layer. If, on the contrary, the sealing tools are too hot, the film tends to adhere to the contact faces of the tools. Dependent on the film thickness and the operating cycle, the parameter range in which a reliable operation is ensured might be extremely narrow. In case of rotary transverse sealing shoes, the sealing period depends from the feeding speed of the film. At high feeding speeds the sealing period is too short to produce a stable sealed seam. Such a boundary speed may be increased by providing that the transverse sealing shoe co-travels along a linear trajectory with the traveling film as described, for example, in International Application WO 96/17720. For this purpose, however, a complex mechanical system is required which often leads to vibrations, wear and operational disturbances.
In general, the sealed seam of a thermoplastic material may be exposed to stresses only after the temperature has dropped below the melting temperature. Since, because of the contact with the hot sealing tool, the entire seam volume is heated, in addition to the speed of the energy supply, the cooling phase also limits the minimum required period to ensure that the sealed seam may be exposed to stresses.
It is an object of the invention to provide an improved apparatus of the above-outlined type with which a rapid sealing may be performed and which is of simple construction.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the apparatus for sealing films together along a path includes a sealing shoe and a counter shoe defining a clearance through which the films pass. The sealing shoe includes an elongated optical energy source having a length dimension oriented generally parallel to the sealing path; a reflector for focussing light emitted by the energy source; a window transparent to the light and having an outer surface adapted to be oriented toward the counter shoe for sealing the films by the light; and a firing arrangement for activating the energy source. The window and the counter shoe are urged toward one another.